Generally speaking, and as is known per se, an optical fiber arc welding operation can be broken down into three main cycles which take place one after another over time, and which are performed with respective arc current and arc duration values that are often predetermined. These cycles take place in succession as follows:
a cleaning cycle during which the ends to be welded together are cleaned; PA1 a pre-fusion cycle during which the ends are pre-fused; and PA1 a fusion proper cycle during which the ends inter-penetrate, melt together, and become united. PA1 during a first step of the cycle, said ends are subjected to a fusion arc and they are caused to inter-penetrate while they are being fused; and PA1 during an "alignment" second step of said cycle, at least one image is taken of said ends, which are not externally illuminated, image analysis and processing is used to calculate an alignment offset between the ends in each image, and the arc fusion is continued so long as the calculated offset remains greater than a predetermined offset threshold; PA1 wherein, during said first step, which is referred to as the "fusion arc current control step", the method further consists in varying the current of said arc by successive increments, in also taking at least one image of the non-externally illuminated ends after each increment, in using image analysis and processing to calculate the brightness of each end, and in stopping the incrementation and the current control step as soon as the calculated brightness of one of the ends reaches a first predetermined threshold value. PA1 it further consists in taking an initial image of the ends prior to said cycle, which ends are externally illuminated, so as to define at least one analysis window on each end, a brightness value to be detected during said cycle being allocated to each analysis window; PA1 it consists in defining two analysis windows on the basis of the initial image, and in performing the current control step in two sequences controlled by the brightness calculated over one of the windows in each end, and then by the brightness calculated over the other one of the windows in each end; PA1 it consists in performing the alignment step under the control of a lateral misalignment calculated on each image of the ends; PA1 it consists in performing the alignment step at the current obtained at the end of the arc control step; and PA1 it further consists in performing a "homogenization" final step during which the ends are homogenized, which step is performed after the alignment step, at an arc current value that is a function of the value obtained at the end of the arc control step, but that is limited to a maximum value.
An article entitled "Real Time Control of Arc Fusion for Optical Fiber Splicing" by W. Zheng, published in the Journal of Lightwave Technology, vol. 11, No. 4, Apr. 1993 describes in particular a control procedure for controlling the fusion cycle during which the ends of the fibers are fused together. That control procedure is based on the fact that the cores of the fibers are made directly visible during fusion. Under those conditions, and by analyzing and processing the digital image taken, it is then possible to calculate the mutual offset between the cores, and to control the final duration of the cycle accordingly so that the surface tension created in the fibers as they are fused together reduces the offset between the cores to a predetermined threshold value.
In that article, the control sequence over time for the fusion cycle starts with an arc which has an initial duration that is defined and is long enough (about 0.5 seconds) to melt the ends of the fibers with a certain amount of inter-penetration taking place, and which has a current that is also defined and is high enough (about 15 mA) to make the cores visible. An image of the cores is then taken. It is analyzed so as to calculate the offset between the cores, while maintaining the arc but reducing its current (to about 10 mA) so as to avoid the effects of surface tension. If the calculated offset is less than a predetermined threshold value, the fusion step is considered to be ended and is stopped. Otherwise, the fusion step continues, with pulses of defined and relatively short duration and with current taking the initial value (15 mA) once again, so that the surface tension in the fibers acts on the offset between the cores, and an image is taken again at the end of each pulse. This process is repeated so long as the offset calculated by analyzing the most-recently taken image is not less the threshold value. In this way, the process defines a core-alignment step.
Such a control sequence is adapted to fibers to be welded together that are "of the same type", i.e. that are of analogous nature or that have analogous thermal characteristics, so that they respond almost identically to the electric arc current. This means that, whenever the type of the fibers to be welded together changes to another type, the initial duration and the current of the fusion arc must be changed, or else separate "adapted" control sequences must be provided, each of which is adapted to one of the different types of the fibers to be welded together.